An organic EL device is a self-emitting device and has been actively studied, since it is light and excellent in the visibility and enables vivid display as compared with liquid-crystal devices.
In 1987, Eastman Kodak's C. W. Tang, et al. developed laminate structure devices in which the constitutive materials share various roles and have thereby put organic EL materials comprising organic materials into practical use. They laminated a fluorescent material capable of transporting electrons and an organic material capable of transporting holes, and thereby injected both charges into a fluorescent material layer for light emission, and obtained a high luminance of at least 1000 cd/m2 at a voltage of 10 V or less (for example, see Patent Reference 1 and Patent Reference 2).    Patent Reference 1: JP-A 8-48656    Patent Reference 2: Japanese Patent No. 3194657
Up to the present, various improvements have been made for practical use of organic EL devices, and an electroluminescent device has come to attain high efficiency and durability, in which various roles are further subdivided to comprise an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer and a cathode, as provided in that order on a substrate (for example, see Non-Patent Reference 1).    Non-Patent Reference 1: Preprint in 9th Seminar of the Japan Society of Applied Physics, pp. 55-61 (2001)
For the purpose of further improvement in luminous efficiency, use of a triplet exciton has been tried, and use of a phosphorescent material is being investigated (for example, see Non-Patent Reference 2).    Non-Patent Reference 2: Preprint in 9th Seminar of the Japan Society of Applied Physics, pp. 23-31 (0.2001)
A light-emitting layer may be formed by doping a charge-transporting compound generally referred to as a host material with a fluorescent material or a phosphorescent material. As described in the above-mentioned seminar preprints, the selection of organic materials in organic EL devices has a significant influence on various characteristics such as the efficiency and the durability of the devices.
In an organic EL device, the charges injected from both electrodes are recombined in the light-emitting layer to emit light. In this, however, since the hole mobility is higher than the electron mobility, there occurs a problem of efficiency reduction owing to passing of a part of holes through the light-emitting layer. Accordingly, an electron-transporting material in which the electron mobility is high is desired.
A typical light-emitting material, tris(8-hydroxyquinoline)aluminium (hereinafter abbreviated as Alq3) generally serves also as an electron-transporting material, but it could not be said that the material may have a hole-blocking ability.
As a measure of preventing the passing of a part of holes through a light-emitting layer and increasing the probability of charge recombination in a light-emitting layer, there is known a method of inserting a hole-blocking layer. As a hole-blocking material, heretofore proposed are triazole derivatives (for example, see Patent Reference 3), bathocuproin (hereinafter abbreviated as BCP), mixed-ligand complexes of aluminium (BAlq) (for example, see Non-Patent Reference 2), etc.
For example, as an electron-transporting material having an excellent hole-blocking ability, proposed is 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter abbreviated as TAZ) (for example, see Patent Reference 3).    Patent Reference 3: Japanese Patent No. 2734341
TAZ has a large work function of 6.6 eV and has a high hole-blocking ability, and is therefore used as an electron-transporting hole-blocking layer to be laminated on the cathode side of the fluorescent light-emitting layer or the phosphorescence-emitting layer formed through vacuum evaporation, coating or the like, and contributes toward increasing the efficiency of organic EL devices (for example, see Non-Patent Reference 3).    Non-Patent Reference 3: Preprint in 28p-A-6 Lecture of the 50th Applied Physics-Associated Joint Lecture Presentation, p. 1413 (2003)
However, TAZ has a serious problem in that its electron transportability is low, and it must be combined with an electron-transporting material having a higher electron transportability in using it for constructing organic EL devices (for example, see Non-Patent Reference 4).    Non-Patent Reference 4: Journal of the Organic Molecule/Bioelectronics Section Committee of the Japan Society of Applied Physics, Vol. 11, No. 1, pp. 13-19 (2000)
BCP has a large work function of 6.7 eV and has a high hole-blocking ability, but has a low glass transition point (Tg) of 83° C., and therefore its film stability is poor, and accordingly, it could not be said that BCP may fully function as a hole-blocking layer. Accordingly, for a phosphorescent device, use of BAlq as the hole-blocking layer is proposed as a measure for life prolongation. The life of the device could be prolonged; however, since the work function of BAlq is 5.8 eV and is small, holes could not be efficiently trapped in the light-emitting layer, and the device could not be said to be sufficient owing to efficiency reduction therein as compared with a device comprising BCP.    Non-Patent Reference 5: 9th Lecture in the Organic Molecule/Bioelectronics Section Committee of the Japan Society of Applied Physics, pp. 23-31 (2001)
All the materials are insufficient in the stability of films thereof or insufficient in the function thereof of blocking holes. For improving the characteristics of organic EL devices, desired are organic compounds showing an excellent electron-injecting/transporting performance, an excellent hole-blocking ability and showing a high stability as thin films.